Identification of a Novel Linear B Cell Epitope on the Sao Protein ofStreptococcus suisSerotype 2

Surface antigen one (Sao) protein is a bacterial surface protein identified in the important zoonotic pathogenStreptococcus suisserotype 2 (S.suis 2) during an extensive search for functional proteins. The Sao protein is anchored to the bacterial cell wall by the LPVTG motif and is widely distributed in manyS.suisserotypes. In this paper, we present the immunodominant epitope peptide of the Sao protein that is recognized by BALB/c antibodies against the Sao protein:(355)SEKQMPSVVNENAVTPEKQMTNKENDNIET(384)(location Sao(355-384)). To determine the core epitope recognized by antibodies, we prepared truncation peptide libraries. Analyses of the immunoreactivity of truncation peptides with anti-Sao(355-384)serum revealed that the most immunoreactive sequence was(355)SEKQMPSVVNENAVTPEK(372)(location Sao(355-372)). Moreover, we observed that this core epitope also showed good specificity based on the ratio of reactivity with serum fromS.suis-positive patients compared to serum fromS.suis-negative patients. Our results point to the potential of using the Sao(355-372)peptide in diagnostic assays to determineS.suisinfection in humans.